1. Field of the Invention
This invention relates generally to the amplification of electrical signals and, more particularly, to wide-band direct-coupled transistor amplifiers.
2. Description of the Related Art
Because of the wide variety of uses for a wide-band direct-coupled amplifier, effort has been expended in improving the design of such devices. One problem that can arise when these amplifiers are implemented with transistors is that the signal in the output stage can vary in a manner that is not related to the an input signal. For example, when transistors in the output stage are subjected to high currents, the temperature of the transistors can be elevated, causing a variation in the transistor parameters. This parameter variation can result in an internally generated change in the output signal. To eliminate these and other parameter changes resulting in similarly undesirable output signal changes, open-loop compensating techniques can be employed. For example, thermally induced temperature changes can be compensated by insertion of thermally coupled components having similar characteristics. Because of aging, difficulty of component characteristic matching and other difficulties, the internally generated signal fluctuations are usually minimized through the use of negative feedback techniques.
Referring to FIG. 1, a wide-band direct-coupled transistor amplifier, according to the prior art, is shown. The amplifier typically includes an input stage 1 for receiving an amplifier input signal, an output stage for receiving a signal (or signals) from the input stage 2 and for providing an amplifier output signal and a feedback stage 3 for taking a signal (or signals) from the output stage 2 and applying the signal or signals to the input stage in a relationship that inhibits undesired fluctuations in the amplifier output signal. FIG. 1 shows the input signal V.sub.IN as being applied to a two pairs of complementary bipolar transistors (Q11 and Q12, Q13 and Q14) and the output signal as the signal developed between a pair of complementary transistors (Q18 and Q19), a common implementation configuration. Also shown (with dotted lines) is a configuration involving R.sub.INPUT and R.sub.REF wherein the amplifier can coupled as a feedback amplifier.
In the related art, the feedback path is typically implemented with voltage amplifiers. For example, in U.S. Pat. No. 4,502,020 entitled Settling Time Reduction in Wide-Band Direct-Coupled Transistor Amplifiers, the bias control circuit of FIG. 1 and FIG. 2 are shown, in FIG. 10, FIG. 11, FIG. 12 and FIG. 13, as being implemented with operational amplifiers. The use of operational amplifiers is both relatively slow and requires careful circuit design and adjustment in order to provide the appropriate voltage levels and voltage translation.
A need has therefore been felt for wide-band direct-coupled transistor amplifier having a feedback path which is relatively simple to implement and which has an expanded frequency response. A need has also been felt for a technique that would permit temperature compensation and voltage off-set correction without complex circuit implementations.